1. Field of the Invention
This invention relates generally to semiconductor device manufacturing. More particularly, the instant invention pertains to methods and apparatus for removal and replacement of individual, defective solder balls on an electronic component.
2. State of the Art
Integrated circuit semiconductor devices (ICs) are small electronic circuits formed on the surface of a wafer or other substrate of semiconductor material such as silicon, gallium arsenide or indium phosphide. The IC devices are fabricated simultaneously in large numbers in wafer form in an array over the active surface of the wafer and tested by a probe to determine electronic characteristics applicable to the intended use of the ICs. The wafer is then subdivided or “singulated” into discrete IC chips or dice, and then further tested, assembled with other components and packaged for customer use through various well-known individual die IC testing and packaging techniques, including leadframe packaging (conventional and leads-over-chip, or LOC), Chip-On-Board (COB) packaging, and flip-chip packaging. Depending upon the relative die and wafer sizes, each wafer is singulated into at least a few dozen dice, as many as several hundred dice, or even as many as several thousand discrete dice when large (such as 30 cm) wafers are employed.
Mechanical and electrical interconnection of discrete semiconductor dice with a carrier substrate, such as a printed circuit board (PCB), is often accomplished with an array of solder balls or bumps projecting from the active surface of the semiconductor die, these external interconnection elements usually having a spherical or near-spherical shape, although other shapes are known. Such a package comprises the aforementioned flip-chip package, so called because the semiconductor die or chip is mounted active surface down over the carrier substrate, supported by the solder balls or bumps. State of the art flip-chip packages may comprise so called “chip-scale” packages, wherein the lateral footprint of the package is the same as, or minimally larger than, the lateral dimension of the semiconductor die itself.
Several methods for forming solder balls or bumps on a workpiece are well known. In the early art, a preformed solder ball was manually placed on a semiconductor die using a forceps or pincer. In later developments, preformed balls have been deposited on bond pads on a semiconductor substrate using a single-ball mounting head or full ball grid array (BGA) mounting head, using vacuum to retain the ball(s) on the head prior to placement on the workpiece. Flux is applied either to the pads or the balls prior to ball placement.
Mounting heads configured to simultaneously apply all balls of a BGA for a semiconductor die are preferred because of savings in labor costs. In the current state of the art, ball grid arrays may even be formed on all of the dice of a full wafer prior to semiconductor die singulation therefrom. Thus, upwards of 10,000 balls may be placed on a wafer prior to the singulation process.
Currently, solder balls may be formed on a workpiece by processes of evaporation, electroplating, stencil printing and serial methods. Each of these processes has particular limitations.
In one version, the solder balls are temporarily fastened to the bond pads of a die by heating to a softening temperature and/or by compression during application. The die with the array of balls placed thereon is then subjected to a thermal “reflow” step to return the balls to substantially spherical shape and then cooled to harden the balls.
In another version, a solder paste preform of any shape may be placed on a metallized bond pad and melted to form a globular or “ball” shape fixedly attached to the bond pad. The ball shape is affected by surface tension of the solder and solder-wettable bond pad or cup-shaped receptacle on the semiconductor die. Alternately, other non-solder wettable passivation materials surrounding a bond pad or receptacle may be utilized to assist in preventing undue solder spread or collapse into adjacent balls (resulting in short-circuits) or damage to the die surface surrounding the balls.
Numerous problems may occur in forming a BGA of a large number of balls on a semiconductor die, wafer or other workpiece, and in the subsequent attachment of the BGA to a carrier substrate. The following discussion pertains to merely a few of such problems.
Where a perforated multiple ball vacuum pickup head is used to simultaneously place all of the BGA's solder balls on a workpiece, a common complaint is that one or more ball-retaining holes is not filled, resulting in workpiece bond pads or other terminal areas devoid of solder balls. In U.S. Pat. No. 4,871,110 to Fukasawa et al., a proposed solution is to provide a second perforated plate above the pickup head to retain the balls therein while sweeping extra balls across the surface to ensure that all holes are filled.
U.S. Pat. No. 5,284,287 to Wilson et al. denotes two problems: nonpickup of solder balls by a multi-ball pickup tool and loss of solder balls while contacting them with flux in a flux bath. In the Wilson patent, solder balls are only partially submerged in the flux and never touch the bottom of the flux bath.
U.S. Pat. No. 5,467,913 to Namekawa et al. discloses a solder ball attachment apparatus in which flux is separately applied to each pad on the semiconductor substrate prior to attaching the solder balls.
U.S. Pat. No. 5,680,984 to Sakemi is directed to a solder ball attachment method using a multi-ball head. The solder balls on the head are dipped in flux prior to placement and reflow. The Sakemi patent notes that when a solder ball is lost in the flux bath, it is recovered in a groove by wiping with a squeegee. No mention is made of what is done to correct the pickup head having an incomplete array of solder balls.
Single-ball pickup heads are known in the art for the purpose of placing solder balls on conductive pads of a workpiece. An example of such is described in U.S. Pat. No. 5,506,385 to Murakami et al. in which vacuum is used to hold a solder ball on a tubular pickup head. While sometimes useful where the number of solder balls on the workpiece is few, its use in forming multi-ball BGAs is contraindicated, being generally very slow, labor-intensive, and expensive. In the Murakami et al. reference, the apparatus uses a spring-biased head which holds a single solder ball, picked up from one of a series of containers holding balls of differing sizes. Flux is applied to each pad, followed by application of a solder ball and thermal reflow resulting from a laser beam focused on the ball.
U.S. Pat. No. 5,695,667 to Eguchi et al. describes an apparatus for forming a BGA of solder balls on a workpiece. A first multi-ball pickup head is utilized to apply the majority of balls to the workpiece. A camera is used to detect empty pads (i.e., having balls missing therefrom). A second, single-ball pickup head is used to fill in empty spaces, and the workpiece is heated in a furnace to reflow all of the solder balls.
Solder balls installed on the workpiece may be defective in various ways. For example, a ball may be undersized (and, thus, not be adequately connected to both a die and the carrier substrate during bonding), or the ball may be oversized (and prevent other adjacent balls from being adequately bonded to the carrier substrate or spread to contact an adjacent ball). The solder ball may also be irregular in shape, resulting in defective bonding. In addition, a solder ball may contain a surface inclusion which prevents or inhibits proper reflow. A solder ball may also be misaligned with its pad, resulting in defective contact therewith. In the current state of the art, such defects are simply dealt with by removing all of the solder balls on a given workpiece and starting over. The “repair” is thus very time-consuming, material-consuming and expensive. None of the above-indicated references appear to recognize or address such problems.
The current emphasis on increased miniaturization and sophistication of integrated circuits has resulted in a continuing reduction in device dimensions, ball diameter and ball spacing (pitch), and increasing numbers of balls in a BGA. As the ball size is decreased, the relative nonuniformity in ball dimensions has been observed to increase. Likewise, as pitch becomes finer, a much greater precision in ball placement is required, inasmuch as lateral ball-to-ball contact must be avoided. The increased numbers of balls required to be transferred to each semiconductor die enhances the opportunity for missed solder balls, extra solder balls, and solder balls outside of the acceptable ranges of size or shape. Thus, the problems indicated hereinabove are exacerbated by the ongoing commercial race to further miniaturize and densify semiconductor devices and the like.
The BGA format has been used with discrete conductive elements other than solder balls, such as conductive epoxy bumps, conductor-filled epoxy bumps and the like, each of which presents its own set of problems. However, solder balls, such as are formed of tin/lead alloy compositions, remain the most widely used conductive elements in BGA constructions. This is primarily because solder is relatively inexpensive and the technologies for ball formation and placement are well developed.
The use of flip-chip technology with solder balls has numerous advantages for interconnection, as compared to conventional leadframe type packages. Flip-chip provides improved electrical performance for high frequency applications such as mainframes and computer workstations. In addition, easier thermal management and reduced susceptibility to electromagnetic interference (EMI) and radiofrequency interference (RFI) emissions are inherent. Furthermore, small solder balls may be densely packed in a BGA array within the footprint of a semiconductor die, which approach conserves surface area (“real estate”) on a carrier substrate and permits a greater number of dice to be placed on a substrate while providing a number of I/Os for each die well in excess of that achievable using leadframes.
Various automation systems have been developed for accurate aligning and joining the solder balls of an installed BGA to the contact sites of a substrate. For example, U.S. Pat. No. 4,899,921 to Bendat et al. discloses a slender optical probe which is inserted between a semiconductor die and a substrate to be joined. Superimposed video images of the die and the substrate permit the two members to be accurately aligned while they are narrowly separated. The probe is retracted and the two members brought together and joined.
In another system disclosed in U.S. Pat. No. 5,894,218 to Farnworth et al., an apparatus for aligning and positioning a die on a temporary test package utilizes video representations of the die surface and the test package to which the die is to be joined.